The present invention relates to a continuous process for the preparation of the tert-butyl ester of an aliphatic C1-C4-carboxylic acid by reacting the carboxylic acid with isobutene.
The novel process leads to tert-butyl esters of aliphatic C1-C4-carboxylic acids. Such esters are widely used. The tert-butyl esters of saturated aliphatic carboxylic acids, such as tert-butyl acetate, are used, for example, as solvents. tert-Butyl esters of (meth)acrylic acid are important starting materials for the preparation of polymers which are used, inter alia, as components of coating dispersions, adhesives or coating resins. tert-Butyl esters are generally prepared by an acid-catalyzed addition reaction of the corresponding carboxylic acids with isobutene (Houben-Weyl, Methoden der Organischen Chemie, Vol. 8, 1952, page 534; U.S. Pat. No. 3,031,495 and U.S. Pat. No. 3,082,246). The catalysts used are acids which are soluble in the reaction mixture, for example mineral acids or alkanesulfonic or arylsulfonic acids (DE-A-12 49 857, U.S. Pat. No. 3,087,962, U.S. Pat. No. 3,088,969), or insoluble catalysts, such as acidic ion-exchange resins (U.S. Pat. No. 3,037,052, U.S. Pat. No. 3,031,495, DE-A-31 05 399, EP-A-268 999).
The reaction of the carboxylic acids with isobutene is carried out as a rule in conventional reaction containers or in columns (DE-A-11 28 428), the thorough mixing of the reaction mixture being effected by stirring or by means of the isobutene stream passed in. The heat is removed in a conventional manner.
The reaction mixture obtained is first freed from the catalyst. With the use of a catalyst soluble in the reaction mixture, this is generally effected by washing with water and/or neutralization with an aqueous alkali solution (DE-A-11 28 428) or by distillation (DE-A-12 49 857). The reaction mixture freed from the catalyst is then worked up by distillation.
As a rule, the following difficulties occur in the preparation of tert-butyl esters:
oligomerization of the isobutene
where xcex1,xcex2-ethylenically unsaturated carboxylic acids are used, polymerization of the carboxylic acids or of the esters under thermal stress
cleavage of the tert-butyl esters under the action of heat and/or in the presence of traces of strong acids
insufficient removal of the heat of reaction occurring in the highly exothermic esterification reaction which may result in cleavage of the ester, oligomerization of the isobutene and, with the use of an xcex1,xcex2-ethylenically unsaturated carboxylic acid, polymerization of the carboxylic acid and of the ester obtained. The last-mentioned effect leads to soiling of the apparatuses, blockage of pipes and pumps and coating of column trays and heat exchanger surfaces, which results in uneconomical and environmentally polluting cleaning of the plants.
In the prior art, numerous attempts have been made to reduce or to avoid the resulting difficulties. Thus, the tendency of isobutene to oligomerize can be reduced by lowering the reaction temperature (U.S. Pat. No. 3,172,905), by the presence of water (U.S. Pat. No. 3,088,969) and by partial neutralization of the catalyst (DE-A 31 05 399). However, these measures have the disadvantage that the reaction rate is reduced.
The formation of isobutene oligomers can also be reduced by using gaseous isobutene (DE-A-11 35 897). However, the disadvantage here is the necessary vaporization of the liquid isoolefins and the handling of large amounts of gas.
In order to avoid or reduce the polymerization of xcex1,xcex2-ethylenically unsaturated compounds, in particular of (meth)acrylic compounds, polymerization inhibitors, such as phenothiazine, hydroquinone or tert-butylpyrocatechol or mixtures thereof, are frequently added, if required with simultaneous addition of air. However, the polymer formation cannot be completely prevented thereby.
In spite of all efforts however, there is no known process to date which reduces the abovementioned difficulties at least to such an extent that an advantageous industrial process results therefrom.
It is an object of the present invention to provide a technically simple, economical and environmentally friendly process for the preparation of tert-butyl esters of aliphatic carboxylic acids.
We have found, surprisingly, that this object is achieved if the reaction of the carboxylic acids with isobutene is carried out in a reactor having a plurality of sections and the reaction temperature in each section is controlled in such a way that it is from 10 to 40xc2x0 C. in the reactor and is advantageously highest in the first section.
The present invention therefore relates to a process for the preparation of the tert-butyl ester of an aliphatic C1-C4-carboxylic acid by reacting the carboxylic acid with isobutene in the liquid phase in the presence of an acidic catalyst and isolating the ester from the reaction mixture obtained, wherein the reaction is carried out in a reactor divided into a plurality of sections and the carboxylic acid, the isobutene and the catalyst are fed into the first zone of the reactor, the reaction temperature in the reactor being controlled so that it is from 10 to 40xc2x0 C. and is highest in the first section.
The aliphatic C1-C4-carboxylic acids are in particular formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, acrylic acid or methacrylic acid, acrylic acid or methacrylic acid being particularly preferred.
The isobutene is fed, preferably in liquid form, into the first section of the reactor. The ratio of carboxylic acid to isobutene may vary within a wide range. Preferably, however, the carboxylic acid is used in a molar excess. The molar ratio of carboxylic acid to isobutene is preferably from 1.1:1 to 1.5:1.